On Orange or T-Mobile.
This phone is a solar powered cellular phone which can be charged in bright sunlight without even connecting to main power supply. The panel that converts the solar energy into the battery is located at the back of the phone. An hour's solar charge allows for 5 to 10 minutes of talk time.
Don't forget the rather epic £7.50 Quidco! Use a fresh basket for each one you order as it's free delivery and Quidco tracks on each sale.
And yes, I know it's not going to save you a huge amount on your electricity bills (especially considering the weather), but it's a nice gimmick at the least. You could take it as a spare phone when you go on holiday (to somewhere sunny, of course!).